Note: Descriptions are shown in the official language in which they were submitted.
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CNT-INFUSED CARBON FIBER MATERIALS AND PROCESS THEREFOR
STATEMENT OF RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Patent Application
No. 11/619,327 filed
January 3, 2007. This application claims priority to U.S. Provisional
Application Nos. 61/168,516,
filed April, 10, 2009, 61/169,055 filed April 14, 2009, 61/155,935 filed
February 27, 2009,
61/157,096 filed March 3, 2009, and 61/182,153 filed May 29, 2009, all of
which are incorporated
herein by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to fiber materials, more specifically to
carbon fiber materials
modified with carbon nanotubes.
BACKGROUND OF THE INVENTION
[0003] Fiber materials are used for many different applications in a wide
variety of industries, such
as the commercial aviation, recreation, industrial and transportation
industries. Commonly-used
fiber materials for these and other applications include carbon fiber,
cellulosic fiber, glass fiber,
metal fiber, ceramic fiber and aramid fiber, for example.
[0004] Carbon fiber is routinely manufactured with sizing agents to protect
the material from
environmental degradation. Additionally, other physical stresses can
compromise carbon fiber
integrity such as compressive forces and self abrasion. Many sizing
formulations used to protect
carbon fibers against these vulnerabilities are proprietary in nature and are
designed to interface with
specific resin types. To realize the benefit of carbon fiber material
properties in a composite, there
must be a good interface between the carbon fibers and the matrix. The sizing
employed on a
carbon fiber can provide a physico-chemical link between fiber and the resin
matrix and thus affects
the mechanical and chemical properties of the composite.
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[0005] However, most conventional sizing agents have a lower interfacial
strength than the carbon
fiber material to which they are applied. As a consequence, the strength of
the sizing and its ability
to withstand interfacial stress ultimately determines the strength of the
overall composite. Thus,
using conventional sizing, the resulting composite will generally have a
strength less than that of the
carbon fiber material.
[0006] It would be useful to develop sizing agents and processes of coating
the same on carbon
fiber materials to address some of the issues described above as well as to
impart desirable
characteristics to the carbon fiber materials. The present invention satisfies
this need and provides
related advantages as well.
SUMMARY OF THE INVENTION
[0007] In some aspects, embodiments disclosed here relate to a composition
that includes a carbon
nanotube (CNT)-infused carbon fiber material. The CNT-infused carbon fiber
material includes a
carbon fiber material of spoolable dimensions and carbon nanotubes (CNTs)
infused to the carbon
fiber material. The infused CNTs are uniform in length and uniform in
distribution. The CNT-
infused carbon fiber material also includes a barrier coating conformally
disposed about the carbon
fiber material, while the CNTs are substantially free of the barrier coating.
[0008] In some aspects, embodiments disclosed herein relatet to a continuous
CNT infusion process
that includes: (a) functionalizing a carbon fiber material; (b) disposing a
barrier coating on the
functionalized carbon fiber material (c) disposing a carbon nanotube (CNT)-
forming catalyst on the
functionalized carbon fiber material; and (d) synthesizing carbon nanotubes,
thereby forming a
carbon nanotube-infused carbon fiber material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Figure 1 shows a transmission electron microscope (TEM) image of a
multi-walled CNT
(MWNT) grown on AS4 carbon fiber via a continuous CVD process.
[0010] Figure 2 shows a TEM image of a double-walled CNT (DWNT) grown on AS4
carbon fiber
via a continuous CVD process.
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[0011] Figure 3 shows a scanning electron microscope (SEM) image of CNTs
growing from within
the barrier coating where the CNT-forming nanoparticle catalyst was
mechanically infused to the
carbon fiber material surface.
[0012] Figure 4 shows a SEM image demonstrating the consistency in length
distribution of CNTs
grown on a carbon fiber material to within 20% of a targeted length of about
40 microns.
[0013] Figure 5 shows an SEM image demonstrating the effect of a barrier
coating on CNT growth.
Dense, well aligned CNTs grew where barrier coating was applied and no CNTs
grew where barrier
coating was absent.
[0014] Figure 6 shows a low magnification SEM of CNTs on carbon fiber
demonstrating the
uniformity of CNT density across the fibers within about 10%.
[0015] Figure 7 shows a process for producing CNT-infused carbon fiber
material in accordance
with the illustrative embodiment of the present invention.
[0016] Figure 8 shows how a carbon fiber material can be infused with CNTs in
a continuous
process to target thermal and electrical conductivity improvements.
[0017] Figure 9 shows how carbon fiber material can be infused with CNTs in a
continuous process
using a "reverse" barrier coating process to target improvements in mechanical
properties, especially
interfacial characteristics such as shear strength.
[0018] Figure 10 shows how carbon fiber material can be infused with CNTs in
another continuous
process using a "hybrid" barrier coating to target improvements in mechanical
properties, especially
interfacial characteristics such as shear strength and interlaminar fracture
toughness.
[0019] Figure 11 shows the effect of infused CNTs on IM7 carbon fiber on
interlaminar fracture
toughness. The baseline material is an unsized IM7 carbon fiber, while the CNT-
Infused material is
an unsized carbon fiber with 15 micron long CNTs infused on the fiber surface.
DETAILED DESCRIPTION
[0020] The present disclosure is directed, in part, to carbon nanotube-infused
("CNT-infused")
carbon fiber materials. The infusion of CNTs to the carbon fiber material can
serve many functions
including, for example, as a sizing agent to protect against damage from
moisture, oxidation,
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abrasion, and compression. A CNT-based sizing can also serve as an interface
between the carbon
fiber material and a matrix material in a composite. The CNTs can also serve
as one of several
sizing agents coating the carbon fiber material.
[0021] Moreover, CNTs infused on a carbon fiber material can alter various
properties of the carbon
fiber material, such as thermal and/or electrical conductivity, and/or tensile
strength, for example.
The processes employed to make CNT-infused carbon fiber materials provide CNTs
with
substantially uniform length and distribution to impart their useful
properties uniformly over the
carbon fiber material that is being modified. Furthermore, the processes
disclosed herein are
suitable for the generation of CNT-infused carbon fiber materials of spoolable
dimensions.
[0022] The present disclosure is also directed, in part, to processes for
making CNT-infused carbon
fiber materials. The processes disclosed herein can be applied to nascent
carbon fiber materials
generated de novo before, or in lieu of, application of a typical sizing
solution to the carbon fiber
material. Alternatively, the processes disclosed herein can utilize a
commercial carbon fiber
material, for example, a carbon tow, that already has a sizing applied to its
surface. In such
embodiments, the sizing can be removed to provide a direct interface between
the carbon fiber
material and the synthesized CNTs, although a barrier coating and/or
transition metal particle can
serve as an intermediate layer providing indirect infusion, as explained
further below. After CNT
synthesis further sizing agents can be applied to the carbon fiber material as
desired.
[0023] The processes described herein allow for the continuous production of
carbon nanotubes of
uniform length and distribution along spoolable lengths of tow, tapes, fabrics
and other 3D woven
structures. While various mats, woven and non-woven fabrics and the like can
be functionalized by
processes of the invention, it is also possible to generate such higher
ordered structures from the
parent tow, yarn or the like after CNT functionalization of these parent
materials. For example, a
CNT-infused woven fabric can be generated from a CNT-infused carbon fiber tow.
[0024] As used herein the term "carbon fiber material" refers to any material
which has carbon fiber
as its elementary structural component. The term encompasses fibers,
filaments, yarns, tows, tows,
tapes, woven and non-woven fabrics, plies, mats, and the like.
[0025] As used herein the term "spoolable dimensions" refers to carbon fiber
materials having at
least one dimension that is not limited in length, allowing for the material
to be stored on a spool or
mandrel. Carbon fiber materials of "spoolable dimensions" have at least one
dimension that
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indicates the use of either batch or continuous processing for CNT infusion as
described herein.
One carbon fiber material of spoolable dimensions that is commercially
available is exemplified by
AS4 12k carbon fiber tow with a tex value of 800 (1 tex = 1 g/1,000m) or 620
yard/lb (Grafil, Inc.,
Sacramento, CA). Commercial carbon fiber tow, in particular, can be obtained
in 5, 10, 20, 50, and
100 lb. (for spools having high weight, usually a 3k/12K tow) spools, for
example, although larger
spools may require special order. Processes of the invention operate readily
with 5 to 20 lb. spools,
although larger spools are usable. Moreover, a pre-process operation can be
incorporated that
divides very large spoolable lengths, for example 100 lb. or more, into easy
to handle dimensions,
such as two 50 lb spools.
[0026] As used herein, the term "carbon nanotube" (CNT, plural CNTs) refers to
any of a number
of cylindrically-shaped allotropes of carbon of the fullerene family including
single-walled carbon
nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), multi-walled carbon
nanotubes
(MWNTs). CNTs can be capped by a fullerene-like structure or open-ended. CNTs
include those
that encapsulate other materials.
[0027] As used herein "uniform in length" refers to length of CNTs grown in a
reactor. "Uniform
length" means that the CNTs have lengths with tolerances of plus or minus
about 20% of the total
CNT length or less, for CNT lengths varying from between about 1 micron to
about 500 microns.
At very short lengths, such as 1-4 microns, this error may be in a range from
between about plus or
minus 20% of the total CNT length up to about plus or minus 1 micron, that is,
somewhat more than
about 20% of the total CNT length.
[0028] As used herein "uniform in distribution" refers to the consistency of
density of CNTs on a
carbon fiber material.. "Uniform distribution" means that the CNTs have a
density on the carbon
fiber material with tolerances of plus or minus about 10% coverage defined as
the percentage of the
surface area of the fiber covered by CNTs. This is equivalent to 1500 CNTs/
m2 for an 8 nm
diameter CNT with 5 walls. Such a figure assumes the space inside the CNTs as
fillable.
[0029] As used herein, the term "infused" means bonded and "infusion" means
the process of
bonding. Such bonding can involve direct covalent bonding, ionic bonding, pi-
pi, and/or van der
Waals force-mediated physisorption. For example, in some embodiments, the CNTs
can be
directely bonded to the carbon fiber material. Bonding can be indirect, such
as the CNT infusion to
the carbon fiber material via a barrier coating and/or an intervening
transition metal nanoparticle
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disposed between the CNTs and carbon fiber material. In the CNT-infused carbon
fiber materials
disclosed herein, the carbon nanotubes can be "infused" to the carbon fiber
material directly or
indirectly as described above. The particular manner in which a CNT is
"infused" to a carbon fiber
materials is referred to as a "bonding motif."
[0030] As used herein, the term "transition metal" refers to any element or
alloy of elements in the
d-block of the periodic table. The term "transition metal" also includes salt
forms of the base
transition metal element such as oxides, carbides, nitrides, and the like.
[0031] As used herein, the term "nanoparticle" or NP (plural NPs), or
grammatical equivalents
thereof refers to particles sized between about 0.1 to about 100 nanometers in
equivalent spherical
diameter, although the NPs need not be spherical in shape. Transition metal
NPs, in particular,
serve as catalysts for CNT growth on the carbon fiber materials.
[0032] As used herein, the term "sizing agent," "fiber sizing agent," or just
"sizing," refers
collectively to materials used in the manufacture of carbon fibers as a
coating to protect the integrity
of carbon fibers, provide enhanced interfacial interactions between a carbon
fiber and a matrix
material in a composite, and/or alter and/or enhance particular physical
properties of a carbon fiber.
In some embodiments, CNTs infused to carbon fiber materials behave as a sizing
agent.
[0033] As used herein, the term "matrix material" refers to a bulk material
than can serve to
organize sized CNT-infused carbon fiber materials in particular orientations,
including random
orientation. The matrix material can benefit from the presence of the CNT-
infused carbon fiber
material by imparting some aspects of the physical and/or chemical properties
of the CNT-infused
carbon fiber material to the matrix material.
[0034] As used herein, the term "material residence time" refers to the amount
of time a discrete
point along a glass fiber material of spoolable dimensions is exposed to CNT
growth conditions
during the CNT infusion processes described herein. This definition includes
the residence time
when employing multiple CNT growth chambers.
[0035] As used herein, the term "linespeed" refers to the speed at which a
glass fiber material of
spoolable dimensions can be fed through the CNT infusion processes described
herein, where
linespeed is a velocity determined by dividing CNT chamber(s) length by the
material residence
time.
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[0036] In some embodiments, the present invention provides a composition that
includes a carbon
nanotube (CNT)-infused carbon fiber material. The CNT-infused carbon fiber
material includes a
carbon fiber material of spoolable dimensions, a barrier coating conformally
disposed about the
carbon fiber material, and carbon nanotubes (CNTs) infused to the carbon fiber
material. The
infusion of CNTs to the carbon fiber material can include a bonding motif of
direct bonding of
individual CNTs to the carbon fiber material or indirect bonding via a
transition metal NP, barrier
coating, or both.
[0037] Without being bound by theory, transition metal NPs, which serve as a
CNT-forming
catalyst, can catalyze CNT growth by forming a CNT growth seed structure. In
one embodiment,
the CNT-forming catalyst can remain at the base of the carbon fiber material,
locked by the barrier
coating, and infused to the surface of the carbon fiber material. In such a
case, the seed structure
initially formed by the transition metal nanoparticle catalyst is sufficient
for continued non-
catalyzed seeded CNT growth without allowing the catalyst to move along the
leading edge of CNT
growth, as often observed in the art. In such a case, the NP serves as a point
of attachment for the
CNT to the carbon fiber material. The presence of the barrier coating can also
lead to further
indirect bonding motifs. For example, the CNT forming catalyst can be locked
into the barrier
coating, as described above, but not in surface contact with carbon fiber
material. In such a case a
stacked structure with the barrier coating disposed between the CNT forming
catalyst and carbon
fiber material results. In either case, the CNTs formed are infused to the
carbon fiber material. In
some embodiments, some barrier coatings will still allow the CNT growth
catalyst to follow the
leading edge of the growing nanotube. In such cases, this can result in direct
bonding of the CNTs
to the carbon fiber material or, optionally, to the barrier coating.
Regardless of the nature of the
actual bonding motif formed between the carbon nanotubes and the carbon fiber
material, the
infused CNT is robust and allows the CNT-infused carbon fiber material to
exhibit carbon nanotube
properties and/or characteristics.
[0038] Again, without being bound by theory, when growing CNTs on carbon fiber
materials, the
elevated temperatures and/or any residual oxygen and/or moisture that can be
present in the reaction
chamber can damage the carbon fiber material. Moreover, the carbon fiber
material itself can be
damaged by reaction with the CNT-forming catalyst itself. That is the carbon
fiber material can
behave as a carbon feedstock to the catalyst at the reaction temperatures
employed for CNT
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synthesis. Such excess carbon can disturb the controlled introduction of the
carbon feedstock gas
and can even serve to poison the catalyst by overloading it with carbon. The
barrier coating
employed in the invention is designed to facilitate CNT synthesis on carbon
fiber materials.
Without being bound by theory, the coating can provide a thermal barrier to
heat degradation and/or
can be a physical barrier preventing exposure of the carbon fiber material to
the environment at the
elevated temperatures. Alternatively or additionally, it can minimize the
surface area contact
between the CNT-forming catalyst and the carbon fiber material and/or it can
mitigate the exposure
of the carbon fiber material to the CNT-forming catalyst at CNT growth
temperatures.
[0039] Compositions having CNT-infused carbon fiber materials are provided in
which the CNTs
are substantially uniform in length. In the continuous process described
herein, the residence time
of the carbon fiber material in a CNT growth chamber can be modulated to
control CNT growth and
ultimately, CNT length. This provides a means to control specific properties
of the CNTs grown.
CNT length can also be controlled through modulation of the carbon feedstock
and carrier gas flow
rates and reaction temperature. Additional control of the CNT properties can
be obtained by
controlling, for example, the size of the catalyst used to prepare the CNTs.
For example, 1 nm
transition metal nanoparticle catalysts can be used to provide SWNTs in
particular. Larger catalysts
can be used to prepare predominantly MWNTs.
[0040] Additionally, the CNT growth processes employed are useful for
providing a CNT-infused
carbon fiber material with uniformly distributed CNTs on carbon fiber
materials while avoiding
bundling and/or aggregation of the CNTs that can occur in processes in which
pre-formed CNTs are
suspended or dispersed in a solvent solution and applied by hand to the carbon
fiber material. Such
aggregated CNTs tend to adhere weakly to a carbon fiber material and the
characteristic CNT
properties are weakly expressed, if at all. In some embodiments, the maximum
distribution density,
expressed as percent coverage, that is, the surface area of fiber covered, can
be as high as about 55%
assuming about 8 nm diameter CNTs with 5 walls. This coverage is calculated by
considering the
space inside the CNTs as being "fillable" space. Various distribution/density
values can be
achieved by varying catalyst dispersion on the surface as well as controlling
gas composition and
process speed. Typically for a given set of parameters, a percent coverage
within about 10% can be
achieved across a fiber surface. Higher density and shorter CNTs are useful
for improving
mechanical properties, while longer CNTs with lower density are useful for
improving thermal and
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electrical properties, although increased density is still favorable. A lower
density can result when
longer CNTs are grown. This can be the result of the higher temperatures and
more rapid growth
causing lower catalyst particle yields.
[0041] The compositions of the invention having CNT-infused carbon fiber
materials can include a
carbon fiber material such as a carbon filament, a carbon fiber yarn, a carbon
fiber tow, a carbon
tape, a carbon fiber-braid, a woven carbon fabric, a non-woven carbon fiber
mat, a carbon fiber ply,
and other 3D woven structures. Carbon filaments include high aspect ratio
carbon fibers having
diameters ranging in size from between about 1 micron to about 100 microns.
Carbon fiber tows are
generally compactly associated bundles of filaments and are usually twisted
together to give yarns.
[0042] Yarns include closely associated bundles of twisted filaments. Each
filament diameter in a
yarn is relatively uniform. Yarns have varying weights described by their
`tex,' expressed as weight
in grams of 1000 linear meters, or denier, expressed as weight in pounds of
10,000 yards, with a
typical tex range usually being between about 200 tex to about 2000 tex.
[0043] Tows include loosely associated bundles of untwisted filaments. As in
yarns, filament
diameter in a tow is generally uniform. Tows also have varying weights and the
tex range is usually
between 200 tex and 2000 tex. They are frequently characterized by the number
of thousands of
filaments in the tow, for example 12K tow, 24K tow, 48K tow, and the like.
[0044] Carbon tapes are materials that can be assembled as weaves or can
represent non-woven
flattened tows. Carbon tapes can vary in width and are generally two-sided
structures similar to
ribbon. Processes of the present invention are compatible with CNT infusion on
one or both sides
of a tape. CNT-infused tapes can resemble a "carpet" or "forest" on a flat
substrate surface. Again,
processes of the invention can be performed in a continuous mode to
functionalize spools of tape.
[0045] Carbon fiber-braids represent rope-like structures of densely packed
carbon fibers. Such
structures can be assembled from carbon yarns, for example. Braided structures
can include a
hollow portion or a braided structure can be assembled about another core
material.
[0046] In some embodiments a number of primary carbon fiber material
structures can be organized
into fabric or sheet-like structures. These include, for example, woven carbon
fabrics, non-woven
carbon fiber mat and carbon fiber ply, in addition to the tapes described
above. Such higher ordered
structures can be assembled from parent tows, yarns, filaments or the like,
with CNTs already
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infused in the parent fiber. Alternatively such structures can serve as the
substrate for the CNT
infusion processes described herein.
[0047] There are three types of carbon fiber which are categorized based on
the precursors used to
generate the fibers, any of which can be used in the invention: Rayon,
Polyacrylonitrile (PAN) and
Pitch. Carbon fiber from rayon precursors, which are cellulosic materials, has
relatively low carbon
content at about 20% and the fibers tend to have low strength and stiffness.
Polyacrylonitrile (PAN)
precursors provide a carbon fiber with a carbon content of about 55%. Carbon
fiber based on a
PAN precursor generally has a higher tensile strength than carbon fiber based
on other carbon fiber
precursors due to a minimum of surface defects.
[0048] Pitch precursors based on petroleum asphalt, coal tar, and polyvinyl
chloride can also be
used to produce carbon fiber. Although pitches are relatively low in cost and
high in carbon yield,
there can be issues of non-uniformity in a given batch.
[0049] CNTs useful for infusion to carbon fiber materials include single-
walled CNTs, double-
walled CNTs, multi-walled CNTs, and mixtures thereof. The exact CNTs to be
used depends on the
application of the CNT-infused carbon fiber. CNTs can be used for thermal
and/or electrical
conductivity applications, or as insulators. In some embodiments, the infused
carbon nanotubes are
single-wall nanotubes. In some embodiments, the infused carbon nanotubes are
multi-wall
nanotubes. In some embodiments, the infused carbon nanotubes are a combination
of single-wall
and multi-wall nanotubes. There are some differences in the characteristic
properties of single-wall
and multi-wall nanotubes that, for some end uses of the fiber, dictate the
synthesis of one or the
other type of nanotube. For example, single-walled nanotubes can be semi-
conducting or metallic,
while multi-walled nanotubes are metallic.
[0050] CNTs lend their characteristic properties such as mechanical strength,
low to moderate
electrical resistivity, high thermal conductivity, and the like to the CNT-
infused carbon fiber
material. For example, in some embodiments, the electrical resistivity of a
carbon nanotube-infused
carbon fiber material is lower than the electrical resistivity of a parent
carbon fiber material. More
generally, the extent to which the resulting CNT-infused fiber expresses these
characteristics can be
a function of the extent and density of coverage of the carbon fiber by the
carbon nanotubes. Any
amount of the fiber surface area, from 0-55% of the fiber can be covered
assuming an 8 nm
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diameter, 5-walled MWNT (again this calculation counts the space inside the
CNTs as fillable).
This number is lower for smaller diameter CNTs and more for greater diameter
CNTs. 55% surface
area coverage is equivalent to about 15,000 CNTs/micron2. Further CNT
properties can be imparted
to the carbon fiber material in a manner dependent on CNT length, as described
above. Infused
CNTs can vary in length ranging from between about 1 micron to about 500
microns, including 1
micron, 2 microns, 3 microns, 4 micron, 5, microns, 6, microns, 7 microns, 8
microns, 9 microns, 10
microns, 15 microns, 20 microns, 25 microns, 30 microns, 35 microns, 40
microns, 45 microns, 50
microns, 60 microns, 70 microns, 80 microns, 90 microns, 100 microns, 150
microns, 200 microns,
250 microns, 300 microns, 350 microns, 400 microns, 450 microns, 500 microns,
and all values in
between. CNTs can also be less than about 1 micron in length, including about
0.5 microns, for
example. CNTs can also be greater than 500 microns, including for example, 510
microns, 520
microns, 550 microns, 600 microns, 700 microns and all values in between.
[0051] Compositions of the invention can incorporate CNTs have a length from
about 1 micron to
about 10 microns. Such CNT lengths can be useful in application to increase
shear strength. CNTs
can also have a length from about 5 to about 70 microns. Such CNT lengths can
be useful in
applications for increased tensile strength if the CNTs are aligned in the
fiber direction. CNTs can
also have a length from about 10 microns to about 100 microns. Such CNT
lengths can be useful to
increase electrical/thermal properties as well as mechanical properties. The
process used in the
invention can also provide CNTs having a length from about 100 microns to
about 500 microns,
which can also be beneficial to increase electrical and thermal properties.
Such control of CNT
length is readily achieved through modulation of carbon feedstock and inert
gas flow rates coupled
with varying linespeeds and growth temperature.
[0052] In some embodiments, compositions that include spoolable lengths of CNT-
infused carbon
fiber materials can have various uniform regions with different lengths of
CNTs. For example, it
can be desirable to have a first portion of CNT-infused carbon fiber material
with uniformly shorter
CNT lengths to enhance shear strength properties, and a second portion of the
same spoolable
material with a uniform longer CNT length to enhance electrical or thermal
properties.
[0053] Processes of the invention for CNT infusion to carbon fiber materials
allow control of the
CNT lengths with uniformity and in a continuous process allowing spoolable
carbon fiber materials
to be functionalized with CNTs at high rates. With material residence times
between 5 to 300
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seconds, linespeeds in a continuous process for a system that is 3 feet long
can be in a range
anywhere from about 0.5 ft/min to about 36 ft/min and greater. The speed
selected depends on
various parameters as explained further below.
[0054] In some embodiments, a material residence time of about 5 to about 30
seconds can produce
CNTs having a length between about 1 micron to about 10 microns. In some
embodiments, a
material residence time of about 30 to about 180 seconds can produce CNTs
having a length
between about 10 microns to about 100 microns. In still further embodiments, a
material residence
time of about 180 to about 300 seconds can produce CNTs having a length
between about 100
microns to about 500 microns. One skilled in the art will recognize that these
ranges are
approximate and that CNT length can also be modulated by reaction
temperatures, and carrier and
carbon feedstock concentrations and flow rates.
[0055] CNT-infused carbon fiber materials of the invention include a barrier
coating. Barrier
coatings can include for example an alkoxysilane, methylsiloxane, an
alumoxane, alumina
nanoparticles, spin on glass and glass nanoparticles. As described below, the
CNT-forming catalyst
can be added to the uncured barrier coating material and then applied to the
carbon fiber material
together. In other embodiments the barrier coating material can be added to
the carbon fiber
material prior to deposition of the CNT-forming catalyst. The barrier coating
material can be of a
thickness sufficiently thin to allow exposure of the CNT-forming catalyst to
the carbon feedstock
for subsequent CVD growth. In some embodiments, the thickness is less than or
about equal to the
effective diameter of the CNT-forming catalyst. In some embodiments, the
thickness of the barrier
coating is in a range from between about 10 nm to about 100 nm. The barrier
coating can also be
less than 10 nm, including lnm, 2 nm, 3nm, 4 nm, 5 rim, 6 rim, 7nm, 8nm, 9 nm,
10 nm, and any
value in between.
[0056] Without being bound by theory, the barrier coating can serve as an
intermediate layer
between the carbon fiber material and the CNTs and serves to mechanically
infuse the CNTs to the
carbon fiber material. Such mechanical infusion still provides a robust system
in which the carbon
fiber material serves as a platform for organizing the CNTs while still
imparting properties of the
CNTs to the carbon fiber material. Moreover, the benefit of including a
barrier coating is the
immediate protection it provides the carbon fiber material from chemical
damage due to exposure to
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moisture and/or any thermal damage due to heating of the carbon fiber material
at the temperatures
used to promote CNT growth.
[0057] The infused CNTs disclosed herein can effectively function as a
replacement for
conventional carbon fiber "sizing." The infused CNTs are more robust than
conventional sizing
materials and can improve the fiber-to-matrix interface in composite materials
and, more generally,
improve fiber-to-fiber interfaces. Indeed, the CNT-infused carbon fiber
materials disclosed herein
are themselves composite materials in the sense the CNT-infused carbon fiber
material properties
will be a combination of those of the carbon fiber material as well as those
of the infused CNTs.
Consequently, embodiments of the present invention provide a means to impart
desired properties to
a carbon fiber material that otherwise lack such properties or possesses them
in insufficient
measure. Carbon fiber materials can be tailored or engineered to meet the
requirements of specific
applications. The CNTs acting as sizing can protect carbon fiber materials
from absorbing moisture
due to the hydrophobic CNT structure. Moreover, hydrophobic matrix materials,
as further
exemplified below, interact well with hydrophobic CNTs to provide improved
fiber to matrix
interactions.
[0058] Despite the beneficial properties imparted to a carbon fiber material
having infused CNTs
described above, the compositions of the present invention can include further
"conventional" sizing
agents. Such sizing agents vary widely in type and function and include, for
example, surfactants,
anti-static agents, lubricants, siloxanes, alkoxysilanes, aminosilanes,
silanes, silanols, polyvinyl
alcohol, starch, and mixtures thereof. Such secondary sizing agents can be
used to protect the CNTs
themselves or provide further properties to the fiber not imparted by the
presence of the infused
CNTs.
[0059] Compositions of the present invention can further include a matrix
material to form a
composite with the CNT-infused carbon fiber material. Such matrix materials
can include, for
example, an epoxy, a polyester, a vinylester, a polyetherimide, a
polyetherketoneketone, a
polyphthalamide, a polyetherketone, a polytheretherketone, a polyimide, a
phenol-formaldehyde,
and a bismaleimide. Matrix materials useful in the present invention can
include any of the known
matrix materials (see Mel M. Schwartz, Composite Materials Handbook (2d ed.
1992)). Matrix
materials more generally can include resins (polymers), both thermosetting and
thermoplastic,
metals, ceramics, and cements.
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[0060] Thermosetting resins useful as matrix materials include phthalic/maelic
type polyesters,
vinyl esters, epoxies, phenolics, cyanates, bismaleimides, and nadic end-
capped polyimides (e.g.,
PMR- 15). Thermoplastic resins include polysulfones, polyamides,
polycarbonates, polyphenylene
oxides, polysulfides, polyether ether ketones, polyether sulfones, polyamide-
imides,
polyetherimides, polyimides, polyarylates, and liquid crystalline polyester.
[0061] Metals useful as matrix materials include alloys of aluminum such as
aluminum 6061, 2024,
and 713 aluminum braze. Ceramics useful as matrix materials include carbon
ceramics, such as
lithium aluminosilicate, oxides such as alumina and mullite, nitrides such as
silicon nitride, and
carbides such as silicon carbide. Cements useful as matrix materials include
carbide-base cermets
(tungsten carbide, chromium carbide, and titanium carbide), refractory cements
(tungsten-thoria and
barium-carbonate-nickel), chromium-alumina, nickel-magnesia iron-zirconium
carbide. Any of the
above-described matrix materials can be used alone or in combination.
[0062] Figure 1-6 shows TEM and SEM images of carbon fiber materials prepared
by the processes
described herein. The procedures for preparing these materials are further
detailed below and in
Examples I-III. Figures 1 and 2 show TEM images of multi walled and double-
walled carbon
nanotubes, respectively, that were prepared on an AS4 carbon fiber in a
continuous process. Figure
3 shows a scanning electron microscope (SEM) image of CNTs growing from within
the barrier
coating after the CNT-forming nanoparticle catalyst was mechanically infused
to a carbon fiber
material surface. Figure 4 shows a SEM image demonstrating the consistency in
length distribution
of CNTs grown on a carbon fiber material to within 20% of a targeted length of
about 40 microns.
Figure 5 shows an SEM image demonstrating the effect of a barrier coating on
CNT growth. Dense,
well aligned CNTs grew where barrier coating was applied and no CNTs grew
where barrier coating
was absent. Figure 6 shows a low magnification SEM of CNTs on carbon fiber
demonstrating the
uniformity of CNT density across the fibers within about 10%.
[0063] CNT-infused carbon fiber materials can be used in a myriad of
applications. For example,
chopped CNT-infused carbon fiber can be used in propellant applications. U.S.
Patent 4,072,546
describes the use of graphite fibers to augment propellant burning rate. The
presence of CNTs
infused on chopped carbon fiber can further enhance such burn rates. CNT-
infused carbon fiber
materials can also be used in flame retardant applications as well. For
example, the CNTs can form
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a protective char layer that retards burning of a material coated with a layer
of CNT infused carbon
fiber material.
[0064] CNT-infused conductive carbon fibers can be used in the manufacture of
electrodes for
superconductors. In the production of superconducting fibers, it can be
challenging to achieve
adequate adhesion of the superconducting layer to a carrier fiber due, in
part, to the different
coefficients of thermal expansion of the fiber material and of the
superconducting layer. Another
difficulty in the art arises during the coating of the fibers by the CVD
process. For example,
reactive gases, such as hydrogen gas or ammonia, can attack the fiber surface
and/or form undesired
hydrocarbon compounds on the fiber surface and make good adhesion of the
superconducting layer
more difficult. CNT-infused carbon fiber materials with barrier coating can
overcome these
aforementioned challenges in the art.
[0065] CNT infused carbon fiber materials can be used in applications
requiring wear-resistance.
U.S. Patent 6,691,393 describes wear resistance in carbon fiber friction
materials. Such carbon fiber
friction materials are used in, for example, automotive brake discs. Other
wear resistance
applications can include, for example, rubber o-rings and gasket seals.
[0066] The large effective surface area of CNTs makes the CNT-infused carbon
fiber materials
effective for water filtration applications and other extractive processes,
such as separation of
organic oils from water. CNT-infused carbon fiber materials can be used to
remove organic toxins
from water tables, water storage facilities, or in-line filters for home and
office use.
[0067] In oilfield technologies, the CNT-infused carbon fibers are useful in
the manufacture of
drilling equipment, such as pipe bearings, piping reinforcement, and rubber o-
rings. Furthermore,
as described above, CNT-infused carbon fibers can be used in extractive
processes. Applying such
extraction properties in a formation containing valuable petroleum deposits,
the CNT-infused
carbon fiber materials can be used to extract oil from otherwise intractable
formations. For
example, the CNT-infuse carbon fiber materials can be used to extract oil from
formations where
substantial water and/or sand is present. The CNT-infused carbon fiber
material can also be useful
to extract heavier oils that would otherwise be difficult to extract due to
their high boiling points. In
conjunction with a perforated piping system, for example, the wicking of such
heavy oils by CNT-
infused carbon materials overcoated on the perforated piping can be
operatively coupled to a
vacuum system, or the like, to continuously remove high boiling fractions from
heavy oil and oil
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shale formations. Moreover, such processes can be used in conjunction with, or
in lieu, of
conventional thermal or catalyzed cracking methods, known in the art.
[0068] CNT-infused carbon fiber materials can enhance structural elements in
aerospace and
ballistics applications. For example, the structures such as nose cones in
missiles, leading edge of
wings, primary structural parts, such as flaps and aerofoils, propellers and
air brakes, small plane
fuselages, helicopter shells and rotor blades, aircraft secondary structural
parts, such as floors, doors,
seats, air conditioners, and secondary tanks and airplane motor parts can
benefit from the structural
enhancement provided by CNT-infused carbon fibers. Structural enhancement in
many other
applications can include, for example, mine sweeper hulls, helmets, radomes,
rocket nozzles, rescue
stretchers, and engine components. In building and construction, structural
enhancement of exterior
features include columns, pediments, domes, cornices, and formwork. Likewise,
in interior building
structures such as blinds, sanitary-ware, window profiles, and the like can
all benefit from the use of
CNT-infused carbon fiber materials.
[0069] In maritime industry, structural enhancement can include boat hulls,
stringers, and decks.
CNT-infused carbon fiber materials can also be used in the heavy
transportation industry in large
panels for trailer walls, floor panels for railcars, truck cabs, exterior body
molding, bus body shells,
and cargo containers, for example. In automotive applications, CNT-infused
carbon fiber materials
can be used in interior parts, such as trimming, seats, and instrument panels.
Exterior structures
such as body panels, openings, underbody, and front and rear modules can all
benefit from the use
of CNT-infused carbon fiber materials. Even automotive engine compartment and
fuel mechanical
area parts, such as axles and suspensions, fuel and exhaust systems, and
electrical and electronic
components can all utilize CNT-infused carbon fiber materials.
[0070] Other applications of CNT-infused carbon fiber materials include,
bridge construction,
reinforced concrete products, such as dowel bars, reinforcing bars, post-
tensioning and pre-stressing
tendons, stay-in-place framework, electric power transmission and distribution
structures such as
utility poles, transmission poles, and cross-arms, highway safety and roadside
features such as sign
supports, guardrails, posts and supports, noise barriers, and in municipal
pipes and storage tanks.
[0071] CNT-infused carbon fiber materials can also be used in a variety of
leisure equipment such
as water and snow skis, kayaks, canoes and paddles, snowboards, golf club
shafts, golf trolleys,
fishing rods, and swimming pools. Other consumer goods and business equipment
include gears,
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pans, housings, gas pressure bottles, components for household appliances,
such as washers,
washing machine drums, dryers, waste disposal units, air conditioners and
humidifiers.
[0072] The electrical properties of CNT-infused carbon fibers also can impact
various energy and
electrical applications. For example, CNT-infused carbon fiber materials can
be used in wind
turbine blades, solar structures, electronic enclosures, such as laptops, cell
phones, computer
cabinets, where such CNT-infused materials can be used in EMI shielding, for
example. Other
applications include powerlines, cooling devices, light poles, circuit boards,
electrical junction
boxes, ladder rails, optical fiber, power built into structures such as data
lines, computer terminal
housings, and business equipment, such as copiers, cash registers and mailing
equipment.
[0073] In some embodiments the present invention provides a continuous process
for CNT infusion
that includes (a) disposing a carbon nanotube-forming catalyst on a surface of
a carbon fiber
material of spoolable dimensions; and (b) synthesizing carbon nanotubes
directly on the carbon fiber
material, thereby forming a carbon nanotube-infused carbon fiber material. For
a 9 foot long
system, the linespeed of the process can range from between about 1.5 ft/min
to about 108 ft/min.
The linespeeds achieved by the process described herein allow the formation of
commercially
relevant quantities of CNT-infused carbon fiber materials with short
production times. For
example, at 36 ft/min linespeed, the quantities of CNT-infused carbon fibers
(over 5% infused
CNTs on fiber by weight) can exceed over 100 pound or more of material
produced per day in a
system that is designed to simultaneously process 5 separate tows (20 lb/tow).
Systems can be made
to produce more tows at once or at faster speeds by repeating growth zones.
Moreover, some steps
in the fabrication of CNTs, as known in the art , have prohibitively slow
rates preventing a
continuous mode of operation. For example, in a typical process known in the
art, a CNT-forming
catalyst reduction step can take 1-12 hours to perform. CNT growth itself can
also be time
consuming, for example requiring tens of minutes for CNT growth, precluding
the rapid linespeeds
realized in the present invention. The process described herein overcomes such
rate limiting steps.
[0074] The CNT-infused carbon fiber material-forming processes of the
invention can avoid CNT
entanglement that occurs when trying to apply suspensions of pre-formed carbon
nanotubes to fiber
materials. That is, because pre-formed CNTs are not fused to the carbon fiber
material, the CNTs
tend to bundle and entangle. The result is a poorly uniform distribution of
CNTs that weakly adhere
to the carbon fiber material. However, processes of the present invention can
provide, if desired, a
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highly uniform entangled CNT mat on the surface of the carbon fiber material
by reducing the
growth density. The CNTs grown at low density are infused in the carbon fiber
material first. In
such embodiments, the fibers do not grow dense enough to induce vertical
alignment, the result is
entangled mats on the carbon fiber material surfaces. By contrast, manual
application of pre-formed
CNTs does not insure uniform distribution and density of a CNT mat on the
carbon fiber material.
[0075] Figure 7 depicts a flow diagram of process 700 for producing CNT-
infused carbon fiber
material in accordance with an illustrative embodiment of the present
invention.
[0076] Process 700 includes at least the operations of:
= 701: Functionalizing the carbon fiber material.
= 702: Applying a barrier coating and a CNT-forming catalyst to the
functionalized
carbon fiber material.
= 704: Heating the carbon fiber material to a temperature that is sufficient
for carbon
nanotube synthesis.
= 706: Promoting CVD-mediated CNT growth on the catalyst-laden carbon fiber.
[0077] In step 701, the carbon fiber material is functionalized to promote
surface wetting of the
fibers and to improve adhesion of the barrier coating.
[0078] To infuse carbon nanotubes into a carbon fiber material, the carbon
nanotubes are
synthesized on the carbon fiber material which is conformally coated with a
barrier coating. In one
embodiment, this is accomplished by first conformally coating the carbon fiber
material with a
barrier coating and then disposing nanotube-forming catalyst on the barrier
coating, as per operation
702. In some embodiments, the barrier coating can be partially cured prior to
catalyst deposition.
This can provide a surface that is receptive to receiving the catalyst and
allowing it to embed in the
barrier coating, including allowing surface contact between the CNT forming
catalyst and the
carbon fiber material. In such embodiments, the barrier coating can be fully
cured after embedding
the catalyst. In some embodiments, the barrier coating is conformally coated
over the carbon fiber
material simultaneously with deposition of the CNT-form catalyst. Once the CNT-
forming catalyst
and barrier coating are in place, the barrier coating can be fully cured.
[0079] In some embodiments, the barrier coating can be fully cured prior to
catalyst deposition. In
such embodiments, a fully cured barrier-coated carbon fiber material can be
treated with a plasma to
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prepare the surface to accept the catalyst. For example, a plasma treated
carbon fiber material
having a cured barrier coating can provide a roughened surface in which the
CNT-forming catalyst
can be deposited. The plasma process for "roughing" the surface of the barrier
thus facilitates
catalyst deposition. The roughness is typically on the scale of nanometers. In
the plasma treatment
process craters or depressions are formed that are nanometers deep and
nanometers in diameter.
Such surface modification can be achieved using a plasma of any one or more of
a variety of
different gases, including, without limitation, argon, helium, oxygen,
nitrogen, and hydrogen. In
some embodiments, plasma roughing can also be performed directly in the carbon
fiber material
itself. This can facilitate adhesion of the barrier coating to the carbon
fiber material.
[0080] As described further below and in conjunction with Figure 7, the
catalyst is prepared as a
liquid solution that contains CNT-forming catalyst that comprise transition
metal nanoparticles.
The diameters of the synthesized nanotubes are related to the size of the
metal particles as described
above. In some embodiments, commercial dispersions of CNT-forming transition
metal
nanoparticle catalyst are available and are used without dilution, in other
embodiments commercial
dispersions of catalyst can be diluted. Whether to dilute such solutions can
depend on the desired
density and length of CNT to be grown as described above.
[0081] With reference to the illustrative embodiment of Figure 7, carbon
nanotube synthesis is
shown based on a chemical vapor deposition (CVD) process and occurs at
elevated temperatures.
The specific temperature is a function of catalyst choice, but will typically
be in a range of about
500 to 1000 C. Accordingly, operation 704 involves heating the barrier-coated
carbon fiber
material to a temperature in the aforementioned range to support carbon
nanotube synthesis.
[0082] In operation 706, CVD-promoted nanotube growth on the catalyst-laden
carbon fiber
material is then performed. The CVD process can be promoted by, for example, a
carbon-
containing feedstock gas such as acetylene, ethylene, and/or ethanol. The CNT
synthesis processes
generally use an inert gas (nitrogen, argon, helium) as a primary carrier gas.
The carbon feedstock is
provided in a range from between about 0% to about 15% of the total mixture. A
substantially inert
environment for CVD growth is prepared by removal of moisture and oxygen from
the growth
chamber.
[0083] In the CNT synthesis process, CNTs grow at the sites of a CNT-forming
transition metal
nanoparticle catalyst. The presence of the strong plasma-creating electric
field can be optionally
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employed to affect nanotube growth. That is, the growth tends to follow the
direction of the electric
field. By properly adjusting the geometry of the plasma spray and electric
field, vertically aligned
CNTs (i. e., perpendicular to the carbon fiber material) can be synthesized.
Under certain conditions,
even in the absence of a plasma, closely-spaced nanotubes will maintain a
vertical growth direction
resulting in a dense array of CNTs resembling a carpet or forest. The presence
of the barrier coating
can also influence the directionality of CNT growth.
[0084] The operation of disposing a catalyst on the carbon fiber material can
be accomplished by
spraying or dip coating a solution or by gas phase deposition via, for
example, a plasma process.
The choice of techniques can be coordinated with the mode with which the
barrier coating is
applied. Thus, in some embodiments, after forming a solution of a catalyst in
a solvent, catalyst can
be applied by spraying or dip coating the barrier coated carbon fiber material
with the solution, or
combinations of spraying and dip coating. Either technique, used alone or in
combination, can be
employed once, twice, thrice, four times, up to any number of times to provide
a carbon fiber
material that is sufficiently uniformly coated with CNT-forming catalyst. When
dip coating is
employed, for example, a carbon fiber material can be placed in a first dip
bath for a first residence
time in the first dip bath. When employing a second dip bath, the carbon fiber
material can be
placed in the second dip bath for a second residence time. For example, carbon
fiber materials can
be subjected to a solution of CNT-forming catalyst for between about 3 seconds
to about 90 seconds
depending on the dip configuration and linespeed. Employing spraying or dip
coating processes, a
carbon fiber material with a surface density of catalyst of less than about 5%
surface coverage to as
high as about 80% coverage, in which the CNT-forming catalyst nanoparticles
are nearly
monolayer. In some embodiments, the process of coating the CNT-forming
catalyst on the carbon
fiber material should produce no more than a monolayer. For example, CNT
growth on a stack of
CNT-forming catalyst can erode the degree of infusion of the CNT to the carbon
fiber material. In
other embodiments, the transition metal catalyst can be deposited on the
carbon fiber material using
evaporation techniques, electrolytic deposition techniques, and other
processes known to those
skilled in the art, such as addition of the transition metal catalyst to a
plasma feedstock gas as a
metal organic, metal salt or other composition promoting gas phase transport.
[0085] Because processes of the invention are designed to be continuous, a
spoolable carbon fiber
material can be dip-coated in a series of baths where dip coating baths are
spatially separated. In a
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continuous process in which nascent carbon fibers are being generated de novo,
dip bath or spraying
of CNT-forming catalyst can be the first step after applying and curing or
partially curing abarrier
coating to the carbon fiber material. Application of the barrier coating and a
CNT-forming catalyst
can be performed in lieu of application of a sizing, for newly formed carbon
fiber materials. In
other embodiments, the CNT-forming catalyst can be applied to newly formed
carbon fibers in the
presence of other sizing agents after barrier coating. Such simultaneous
application of CNT-
forming catalyst and other sizing agents can still provide the CNT-forming
catalyst in surface
contact with the barrier coating of the carbon fiber material to insure CNT
infusion.
[0086] The catalyst solution employed can be a transition metal nanoparticle
which can be any d
block transition metal as described above. In addition, the nanoparticles can
include alloys and non-
alloy mixtures of d-block metals in elemental form or in salt form, and
mixtures thereof. Such salt
forms include, without limitation, oxides, carbides, and nitrides. Non-
limiting exemplary transition
metal NPs include Ni, Fe, Co, Mo, Cu, Pt, Au, and Ag and salts thereof and
mixtures thereof. In
some embodiments, such CNT-forming catalysts are disposed on the carbon fiber
by applying or
infusing a CNT-forming catalyst directly to the carbon fiber material
simultaneously with barrier
coating deposition. Many of these transition metal catalysts are readily
commercially available from
a variety of suppliers, including, for example, Ferrotec Corporation (Bedford,
NH).
[0087] Catalyst solutions used for applying the CNT-forming catalyst to the
carbon fiber material
can be in any common solvent that allows the CNT-forming catalyst to be
uniformly dispersed
throughout. Such solvents can include, without limitation, water, acetone,
hexane, isopropyl
alcohol, toluene, ethanol, methanol, tetrahydrofuran (THF), cyclohexane or any
other solvent with
controlled polarity to create an appropriate dispersion of the CNT-forming
catalyst nanoparticles.
Concentrations of CNT-forming catalyst can be in a range from about 1:1 to
1:10000 catalyst to
solvent. Such concentrations can be used when the barrier coating and CNT-
forming catalyst is
applied simultaneously as well.
[0088] In some embodiments heating of the carbon fiber material can be at a
temperature that is
between about 500 C and 1000 C to synthesize carbon nanotubes after
deposition of the CNT-
forming catalyst. Heating at these temperatures can be performed prior to or
substantially
simultaneously with introduction of a carbon feedstock for CNT growth.
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[0089] In some embodiments, the present invention provides a process that
includes removing
sizing agents from a carbon fiber material, applying a barrier coating
conformally over the carbon
fiber material, applying a CNT-forming catalyst to the carbon fiber material,
heating the carbon
fiber material to at least 500 C, and synthesizing carbon nanotubes on the
carbon fiber material. In
some embodiments, operations of the CNT-infusion process include removing
sizing from a carbon
fiber material, applying a barrier coating to the carbon fiber material,
applying a CNT-forming
catalyst to the carbon fiber, heating the fiber to CNT-synthesis temperature
and CVD-promoted
CNT growth the catalyst-laden carbon fiber material. Thus, where commercial
carbon fiber
materials are employed, processes for constructing CNT-infused carbon fibers
can include a discrete
step of removing sizing from the carbon fiber material before disposing
barrier coating and the
catalyst on the carbon fiber material.
[0090] The step of synthesizing carbon nanotubes can include numerous
techniques for forming
carbon nanotubes, including those disclosed in co-pending U.S. Patent
Application No. US
2004/0245088 which is incorporated herein by reference. The CNTs grown on
fibers of the present
invention can be accomplished by techniques known in the art including,
without limitation, micro-
cavity, thermal or plasma-enhanced CVD techniques, laser ablation, arc
discharge, and high
pressure carbon monoxide (HiPCO). During CVD, in particular, a barrier coated
carbon fiber
material with CNT-forming catalyst disposed thereon, can be used directly. In
some embodiments,
any conventional sizing agents can be removed prior CNT synthesis. In some
embodiments,
acetylene gas is ionized to create a jet of cold carbon plasma for CNT
synthesis. The plasma is
directed toward the catalyst-bearing carbon fiber material. Thus, in some
embodiments synthesizing
CNTs on a carbon fiber material includes (a) forming a carbon plasma; and (b)
directing the carbon
plasma onto the catalyst disposed on the carbon fiber material. The diameters
of the CNTs that are
grown are dictated by the size of the CNT-forming catalyst as described above.
In some
embodiments, the sized fiber substrate is heated to between about 550 to about
800 C to facilitate
CNT synthesis. To initiate the growth of CNTs, two gases are bled into the
reactor: a process gas
such as argon, helium, or nitrogen, and a carbon-containing gas, such as
acetylene, ethylene, ethanol
or methane. CNTs grow at the sites of the CNT-forming catalyst.
[0091] In some embodiments, the CVD growth is plasma-enhanced. A plasma can be
generated by
providing an electric field during the growth process. CNTs grown under these
conditions can
follow the direction of the electric field. Thus, by adjusting the geometry of
the reactor vertically
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aligned carbon nanotubes can be grown radially about a cylindrical fiber. In
some embodiments, a
plasma is not required for radial growth about the fiber. For carbon fiber
materials that have distinct
sides such as tapes, mats, fabrics, plies, and the like, catalyst can be
disposed on one or both sides
and correspondingly, CNTs can be grown on one or both sides as well.
[0092] As described above, CNT-synthesis is performed at a rate sufficient to
provide a continuous
process for functionalizing spoolable carbon fiber materials. Numerous
apparatus configurations
faciliate such continuous synthesis as exemplified below.
[0093] In some embodiments, CNT-infused carbon fiber materials can be
constructed in an "all
plasma" process. An all plasma process can being with roughing the carbon
fiber material with a
plasma as described above to improve fiber surface wetting characteristics and
provide a more
conformal barrier coating, as well as improve coating adhesion via mechanical
interlocking and
chemical adhesion through the use of functionalization of the carbon fiber
material by using specific
reactive gas species, such as oxygen, nitrogen, hydrogen in argon or helium
based plasmas.
[0094] Barrier coated carbon fiber materials pass through numerous further
plasma-mediated steps
to form the final CNT-infused product. In some embodiments, the all plasma
process can include a
second surface modification after the barrier coating is cured. This is a
plasma process for
"roughing" the surface of the barrier coating on the carbon fiber material to
facilitate catalyst
deposition. As described above, surface modification can be achieved using a
plasma of any one or
more of a variety of different gases, including, without limitation, argon,
helium, oxygen, ammonia,
hydrogen, and nitrogen.
[0095] After surface modification, the barrier coated carbon fiber material
proceeds to catalyst
application. This is a plasma process for depositing the CNT-forming catalyst
on the fibers. The
CNT-forming catalyst is typically a transition metal as described above. The
transition metal
catalyst can be added to a plasma feedstock gas as a precursor in the form of
a ferrofluid, a metal
organic, metal salt or other composition for promoting gas phase transport.
The catalyst can be
applied at room temperature in the ambient environment with neither vacuum nor
an inert
atmosphere being required. In some embodiments, the carbon fiber material is
cooled prior to
catalyst application.
[0096] Continuing the all-plasma process, carbon nanotube synthesis occurs in
a CNT-growth
reactor. This can be achieved through the use of plasma-enhanced chemical
vapor deposition,
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wherein carbon plasma is sprayed onto the catalyst-laden fibers. Since carbon
nanotube growth
occurs at elevated temperatures (typically in a range of about 500 to 1000, C
depending on the
catalyst), the catalyst laden fibers can be heated prior to exposing to the
carbon plasma. For the
infusion process, the carbon fiber material can be optionally heated until it
softens. After heating,
the carbon fiber material is ready to receive the carbon plasma. The carbon
plasma is generated, for
example, by passing a carbon containing gas such as acetylene, ethylene,
ethanol, and the like,
through an electric field that is capable of ionizing the gas. This cold
carbon plasma is directed, via
spray nozzles, to the carbon fiber material. The carbon fiber material can be
in close proximity to
the spray nozzles, such as within about 1 centimeter of the spray nozzles, to
receive the plasma. In
some embodiments, heaters are disposed above the carbon fiber material at the
plasma sprayers to
maintain the elevated temperature of the carbon fiber material.
[0097] Another configuration for continuous carbon nanotube synthesis involves
a special
rectangular reactor for the synthesis and growth of carbon nanotubes directly
on carbon fiber
materials. The reactor can be designed for use in a continuous in-line process
for producing carbon-
nanotube bearing fibers. In some embodiments, CNTs are grown via a chemical
vapor deposition
("CVD") process at atmospheric pressure and at elevated temperature in the
range of about 550 C
to about 800 C in a multi-zone reactor. The fact that the synthesis occurs at
atmospheric pressure
is one factor that facilitates the incorporation of the reactor into a
continuous processing line for
CNT-on-fiber synthesis. Another advantage consistent with in-line continuous
processing using
such a zone reactor is that CNT growth occurs in a seconds, as opposed to
minutes (or longer) as in
other procedures and apparatus configurations typical in the art.
[0098] CNT synthesis reactors in accordance with the various embodiments
include the following
features:
[0099] Rectangular Configured Synthesis Reactors: The cross section of a
typical CNT synthesis
reactor known in the art is circular. There are a number of reasons for this
including, for example,
historical reasons (cylindrical reactors are often used in laboratories) and
convenience (flow
dynamics are easy to model in cylindrical reactors, heater systems readily
accept circular tubes
(quartz, etc.), and ease of manufacturing. Departing from the cylindrical
convention, the present
invention provides a CNT synthesis reactor having a rectangular cross section.
The reasons for the
departure are as follows: 1. Since many carbon fiber materials that can be
processed by the reactor
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are relatively planar such as flat tape or sheet-like in form, a circular
cross section is an inefficient
use of the reactor volume. This inefficiency results in several drawbacks for
cylindrical CNT
synthesis reactors including, for example, a) maintaining a sufficient system
purge; increased
reactor volume requires increased gas flow rates to maintain the same level of
gas purge. This
results in a system that is inefficient for high volume production of CNTs in
an open environment;
b) increased carbon feedstock gas flow; the relative increase in inert gas
flow, as per a) above,
requires increased carbon feedstock gas flows. Consider that the volume of a
12K carbon fiber tow
is 2000 times less than the total volume of a synthesis reactor having a
rectangular cross section. In
an equivalent growth cylindrical reactor (i.e., a cylindrical reactor that has
a width that
accommodates the same planarized carbon fiber material as the rectangular
cross-section reactor),
the volume of the carbon fiber material is 17,500 times less than the volume
of the chamber.
Although gas deposition processes, such as CVD, are typically governed by
pressure and
temperature alone, volume has a significant impact on the efficiency of
deposition. With a
rectangular reactor there is a still excess volume. This excess volume
facilitates unwanted
reactions; yet a cylindrical reactor has about eight times that volume. Due to
this greater
opportunity for competing reactions to occur, the desired reactions
effectively occur more slowly in
a cylindrical reactor chamber. Such a slow down in CNT growth, is problematic
for the
development of a continuous process. One benefit of a rectangular reactor
configuration is that the
reactor volume can be decreased by using a small height for the rectangular
chamber to make this
volume ratio better and reactions more efficient. In some embodiments of the
present invention, the
total volume of a rectangular synthesis reactor is no more than about 3000
times greater than the
total volume of a carbon fiber material being passed through the synthesis
reactor. In some further
embodiments, the total volume of the rectangular synthesis reactor is no more
than about 4000 times
greater than the total volume of the carbon fiber material being passed
through the synthesis reactor.
In some still further embodiments, the total volume of the rectangular
synthesis reactor is less than
about 10,000 times greater than the total volume of the carbon fiber material
being passed through
the synthesis reactor. Additionally, it is notable that when using a
cylindrical reactor, more carbon
feedstock gas is required to provide the same flow percent as compared to
reactors having a
rectangular cross section. It should be appreciated that in some other
embodiments, the synthesis
reactor has a cross section that is described by polygonal forms that are not
rectangular, but are
relatively similar thereto and provide a similar reduction in reactor volume
relative to a reactor
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having a circular cross section; c) problematic temperature distribution; when
a relatively small-
diameter reactor is used, the temperature gradient from the center of the
chamber to the walls
thereof is minimal. But with increased size, such as would be used for
commercial-scale
production, the temperature gradient increases. Such temperature gradients
result in product quality
variations across a carbon fiber material substrate (i.e., product quality
varies as a function of radial
position). This problem is substantially avoided when using a reactor having a
rectangular cross
section. In particular, when a planar substrate is used, reactor height can be
maintained constant as
the size of the substrate scales upward. Temperature gradients between the top
and bottom of the
reactor are essentially negligible and, as a consequence, thermal issues and
the product-quality
variations that result are avoided. 2. Gas introduction: Because tubular
furnaces are normally
employed in the art, typical CNT synthesis reactors introduce gas at one end
and draw it through the
reactor to the other end. In some embodiments disclosed herein, gas can be
introduced at the center
of the reactor or within a target growth zone, symmetrically, either through
the sides or through the
top and bottom plates of the reactor. This improves the overall CNT growth
rate because the
incoming feedstock gas is continuously replenishing at the hottest portion of
the system, which is
where CNT growth is most active. This constant gas replenishment is an
important aspect to the
increased growth rate exhibited by the rectangular CNT reactors.
[00100] Zoning. Chambers that provide a relatively cool purge zone depend from
both ends
of the rectangular synthesis reactor. Applicants have determined that if hot
gas were to mix with the
external environment (i.e., outside of the reactor), there would be an
increase in degradation of the
carbon fiber material. The cool purge zones provide a buffer between the
internal system and
external environments. Typical CNT synthesis reactor configurations known in
the art typically
require that the substrate is carefully (and slowly) cooled. The cool purge
zone at the exit of the
present rectangular CNT growth reactor achieves the cooling in a short period
of time, as required
for the continuous in-line processing.
[00101] Non-contact, hot-walled, metallic reactor. In some embodiments, a hot-
walled
reactor is made of metal is employed, in particular stainless steel. This may
appear counterintuitive
because metal, and stainless steel in particular, is more susceptible to
carbon deposition (i.e., soot
and by-product formation). Thus, most CNT reactor configurations use quartz
reactors because
there is less carbon deposited, quartz is easier to clean, and quartz
facilitates sample observation.
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However, Applicants have observed that the increased soot and carbon
deposition on stainless steel
results in more consistent, faster, more efficient, and more stable CNT
growth. Without being
bound by theory it has been indicated that, in conjunction with atmospheric
operation, the CVD
process occurring in the reactor is diffusion limited. That is, the catalyst
is "overfed;" too much
carbon is available in the reactor system due to its relatively higher partial
pressure (than if the
reactor was operating under partial vacuum). As a consequence, in an open
system - especially a
clean one - too much carbon can adhere to catalyst particles, compromising
their ability to
synthesize CNTs. In some embodiments, the rectangular reactor is intentionally
run when the
reactor is "dirty," that is with soot deposited on the metallic reactor walls.
Once carbon deposits to
a monolayer on the walls of the reactor, carbon will readily deposit over
itself. Since some of the
available carbon is "withdrawn" due to this mechanism, the remaining carbon
feedstock, in the form
of radicals, react with the catalyst at a rate that does not poison the
catalyst. Existing systems run
"cleanly" which, if they were open for continuous processing, would produced a
much lower yield
of CNTs at reduced growth rates.
[00102] Although it is generally beneficial to perform CNT synthesis "dirty"
as described
above, certain portions of the apparatus, such as gas manifolds and inlets,
can nonetheless
negatively impact the CNT growth process when soot created blockages. In order
to combat this
problem, such areas of the CNT growth reaction chamber can be protected with
soot inhibiting
coatings such as silica, alumina, or MgO. In practice, these portions of the
apparatus can be dip-
coated in these soot inhibiting coatings. Metals such as INVAR can be used
with these coatings
as INVAR has a similar CTE (coefficient of thermal expansion) ensuring proper
adhesion of the
coating at higher temperatures, preventing the soot from significantly
building up in critical zones.
[00103] Combined Catalyst Reduction and CNT Synthesis. In the CNT synthesis
reactor
disclosed herein, both catalyst reduction and CNT growth occur within the
reactor. This is
significant because the reduction step cannot be accomplished timely enough
for use in a continuous
process if performed as a discrete operation. In a typical process known in
the art, a reduction step
typically takes 1-12 hours to perform. Both operations occur in a reactor in
accordance with the
present invention due, at least in part, to the fact that carbon feedstock gas
is introduced at the center
of the reactor, not the end as would be typical in the art using cylindrical
reactors. The reduction
process occurs as the fibers enter the heated zone; by this point, the gas has
had time to react with
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the walls and cool off prior to reacting with the catalyst and causing the
oxidation reduction (via
hydrogen radical interactions). It is this transition region where the
reduction occurs. At the hottest
isothermal zone in the system, the CNT growth occurs, with the greatest growth
rate occurring
proximal to,the gas inlets near the center of the reactor.
[00104] In some embodiments, when loosely affiliated carbon fiber materials,
such as carbon
tow are employed, the continuous process can include steps that spreads out
the strands and/or
filaments of the tow. Thus, as a tow is unspooled it can be spread using a
vacuum-based fiber
spreading system, for example. When employing sized carbon fibers, which can
be relatively stiff,
additional heating can be employed in order to "soften" the tow to facilitate
fiber spreading. The
spread fibers which comprise individual filaments can be spread apart
sufficiently to expose an
entire surface area of the filaments, thus allowing the tow to more
efficiently react in subsequent
process steps. Such spreading can approach between about 4 inches to about 6
inches across for a
3k tow. The spread carbon tow can pass through a surface treatment step that
is composed of a
plasma system as described above. After a barrier coating is applied and
roughened, spread fibers
then can pass through a CNT-forming catalyst dip bath. The result is fibers of
the carbon tow that
have catalyst particles distributed radially on their surface. The catalyzed
laden fibers of the tow
then enter an appropriate CNT growth chamber, such as the rectangular chamber
described above,
where a flow through atmospheric pressure CVD or PE-CVD process is used to
synthesize the
CNTs at rates as high as several microns per second. The fibers of the tow,
now with radially
aligned CNTs, exit the CNT growth reactor.
[00105] In some embodiments, CNT-infused carbon fiber materials can pass
through yet
another treatment process that, in some embodiments is a plasma process used
to functionalize the
CNTs. Additional functionalization of CNTs can be used to promote their
adhesion to particular
resins. Thus, in some embodiments, the present invention provides CNT-infused
carbon fiber
materials having functionalized CNTs.
[00106] As part of the continuous processing of spoolable carbon fiber
materials, the a CNT-
infused carbon fiber material can further pass through a sizing dip bath to
apply any additional
sizing agents which can be beneficial in a final product. Finally if wet
winding is desired, the CNT-
infused carbon fiber materials can be passed through a resin bath and wound on
a mandrel or spool.
The resulting carbon fiber material/resin combination locks the CNTs on the
carbon fiber material
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allowing for easier handling and composite fabrication. In some embodiments,
CNT infusion is
used to provide improved filament winding. Thus, CNTs formed on carbon fibers
such as carbon
tow, are passed through a resin bath to produce resin-impregnated, CNT-infused
carbon tow. After
resin impregnation, the carbon tow can be positioned on the surface of a
rotating mandrel by a
delivery head. The tow can then be wound onto the mandrel in a precise
geometric pattern in
known fashion.
[00107] The winding process described above provides pipes, tubes, or other
forms as are
characteristically produced via a male mold. But the forms made from the
winding process
disclosed herein differ from those produced via conventional filament winding
processes.
Specifically, in the process disclosed herein, the forms are made from
composite materials that
include CNT-infused tow. Such forms will therefore benefit from enhanced
strength and the like, as
provided by the CNT-infused tow.
[00108] In some embodiments, a continuous process for infusion of CNTs on
spoolable
carbon fiber materials can achieve a linespeed between about 0.5 ft/min to
about 36 ft/min. In this
embodiment where the CNT growth chamber is 3 feet long and operating at a 750
C growth
temperature, the process can be run with a linespeed of about 6 ft/min to
about 36 ft/min to produce,
for example, CNTs having a length between about 1 micron to about 10 microns.
The process can
also be run with a linespeed of about 1 ft/min to about 6 ft/min to produce,
for example, CNTs
having a length between about 10 microns to about 100 microns. The process can
be run with a
linespeed of about 0.5 ft/min to about 1 ft/min to produce, for example, CNTs
having a length
between about 100 microns to about 200 microns. The CNT length is not tied
only to linespeed and
growth temperature, however, the flow rate of both the carbon feedstock and
the inert carrier gases
can also influence CNT length. For example, a flow rate consisting of less
than 1 % carbon
feedstock in inert gas at high linespeeds (6 ft/min to 36 ft/min) will result
in CNTs having a length
between 1 micron to about 5 microns. A flow rate consisting of more than I%
carbon feedstock in
inert gas at high linespeeds (6 ft/min to 36 ft/min) will result in CNTs
having length between 5
microns to about 10 microns.
[00109] In some embodiments, more than one carbon material can be run
simultaneously
through the process. For example, multiple tapes tows, filaments, strand and
the like can be run
through the process in parallel. Thus, any number of pre-fabricated spools of
carbon fiber material
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can be run in parallel through the process and re-spooled at the end of the
process. The number of
spooled carbon fiber materials that can be run in parallel can include one,
two, three, four, five, six,
up to any number that can be accommodated by the width of the CNT-growth
reaction chamber.
Moreover, when multiple carbon fiber materials are run through the process,
the number of
collection spools can be less than the number of spools at the start of the
process. In such
embodiments, carbon strands, tows, or the like can be sent through a further
process of combining
such carbon fiber materials into higher ordered carbon fiber materials such as
woven fabrics or the
like. The continuous process can also incorporate a post processing chopper
that facilitates the
formation CNT-infused chopped fiber mats, for example.
[00110] In some embodiments, processes of the invention allow for synthesizing
a first
amount of a first type of carbon nanotube on the carbon fiber material, in
which the first type of
carbon nanotube is selected to alter at least one first property of the carbon
fiber material.
Subsequently, process of the invention allow for synthesizing a second amount
of a second type of
carbon nanotube on the carbon fiber material, in which the second type of
carbon nanotube is
selected to alter at least one second property of the carbon fiber material.
[00111] In some embodiments, the first amount and second amount of CNTs are
different.
This can be accompanied by a change in the CNT type or not. Thus, varying the
density of CNTs
can be used to alter the properties of the original carbon fiber material,
even if the CNT type
remains unchanged. CNT type can include CNT length and the number of walls,
for example. In
some embodiments the first amount and the second amount are the same. If
different properties are
desirable in this case along the two different stretches of the spoolable
material, then the CNT type
can be changed, such as the CNT length. For example, longer CNTs can be useful
in
electrical/thermal applications, while shorter CNTs can be useful in
mechanical strengthening
applications.
[00112] In light of the aforementioned discussion regarding altering the
properties of the
carbon fiber materials, the first type of carbon nanotube and the second type
of carbon nanotube can
be the same, in some embodiments, while the first type of carbon nanotube and
the second type of
carbon nanotube can be different, in other embodiments. Likewise, the first
property and the second
property can be the same, in some embodiments. For example, the EMI shielding
property can be
the property of interest addressed by the first amount and type of CNTs and
the 2nd amount and
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type of CNTs, but the degree of change in this property can be different, as
reflected by differing
amounts, and/or types of CNTs employed. Finally, in some embodiments, the
first property and the
second property can be different. Again this may reflect a change in CNT type.
For example the
first property can be mechanical strength with shorter CNTs, while the second
property can be
electrical/thermal properties with longer CNTs. One skilled in the art will
recognize the ability to
tailor the properties of the carbon fiber material through the use of
different CNT densities, CNT
lengths, and the number of walls in the CNTs, such as single-walled, double-
walled, and multi-
walled, for example.
[00113] In some embodiments, processes of the present invention provides
synthesizing a
first amount of carbon nanotubes on a carbon fiber material, such that this
first amount allows the
carbon nanotube-infused carbon fiber material to exhibit a second group of
properties that differ
from a first group of properties exhibited by the carbon fiber material
itself. That is, selecting an
amount that can alter one or more properties of the carbon fiber material,
such as tensile strength.
The first group of properties and second group of properties can include at
least one of the same
properties, thus representing enhancing an already existing property of the
carbon fiber material. In
some embodiments, CNT infusion can impart a second group of properties to the
carbon nanotube-
infused carbon fiber material that is not included among the first group of
properties exhibited by
the carbon fiber material itself.
[00114] In some embodiments, a first amount of carbon nanotubes is selected
such that the
value of at least one property selected from the group consisting of tensile
strength, Young's
Modulus, shear strength, shear modulus, toughness, compression strength,
compression modulus,
density, EM wave absorptivity/reflectivity, acoustic transmittance, electrical
conductivity, and
thermal conductivity of the carbon nanotube-infused carbon fiber material
differs from the value of
the same property of the carbon fiber material itself.
[00115] Tensile strength can include three different measurements: 1) Yield
strength which
evaluates the stress at which material strain changes from elastic deformation
to plastic deformation,
causing the material to deform permanently; 2) Ultimate strength which
evaluates the maximum
stress a material can withstand when subjected to tension, compression or
shearing; and 3) Breaking
strength which evaluates the stress coordinate on a stress-strain curve at the
point of rupture.
Composite shear strength evaluates the stress at which a material fails when a
load is applied
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perpendicular to the fiber direction. Compression strength evaluates the
stress at which a material
fails when a compressive load is applied.
[00116] Multiwalled carbon nanotubes, in particular, have the highest tensile
strength of any
material yet measured, with a tensile strength of 63 GPa having been achieved.
Moreover,
theoretical calculations have indicated possible tensile strengths of CNTs of
about 300 GPa. Thus,
CNT-infused carbon fiber materials are expected to have substantially higher
ultimate strength
compared to the parent carbon fiber material. As described above, the increase
in tensile strength
will depend on the exact nature of the CNTs used as well as the density and
distribution on the
carbon fiber material. CNT-infused carbon fiber materials can exhibit a tow to
three times increase
in tensile properties, for example. Exemplary CNT-infused carbon fiber
materials can have as high
as three times the shear strength as the parent unfunctionalized carbon fiber
material and as high as
2.5 times the compression strength.
[00117] Young's modulus is a measure of the stiffness of an isotropic elastic
material. It is
defined as the ratio of the uniaxial stress over the uniaxial strain in the
range of stress in which
Hooke's Law holds. This can be experimentally determined from the slope of a
stress-strain curve
created during tensile tests conducted on a sample of the material.
[00118] Electrical conductivity or specific conductance is a measure of a
material's ability to
conduct an electric current. CNTs with particular structural parameters such
as the degree of twist,
which relates to CNT chirality, can be highly conducting, thus exhibiting
metallic properties. A
recognized system of nomenclature (M. S. Dresselhaus, et al. Science of
Fullerenes and Carbon
Nanotubes, Academic Press, San Diego, CA pp. 756-760, (1996)) has been
formalized and is
recognized by those skilled in the art with respect to CNT chirality. Thus,
for example, CNTs are
distinguished from each other by a double index (n,m) where n and in are
integers that describe the
cut and wrapping of hexagonal graphite so that it makes a tube when it is
wrapped onto the surface
of a cylinder and the edges are sealed together. When the two indices are the
same, m=n, the
resultant tube is said to be of the "arm-chair" (or n,n) type, since when the
tube is cut perpendicular
to the CNT axis only the sides of the hexagons are exposed and their pattern
around the periphery of
the tube edge resembles the arm and seat of an arm chair repeated n times. Arm-
chair CNTs, in
particular SWNTs, are metallic, and have extremely high electrical and thermal
conductivity. In
addition, such S WNTs have-extremely high tensile strength.
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[00119] In addition to the degree of twist CNT diameter also effects
electrical conductivity.
As described above, CNT diameter can be controlled by use of controlled size
CNT-forming
catalyst nanoparticles. CNTs can also be formed as semi-conducting materials.
Conductivity in
multi-walled CNTs (MWNTs) can be more complex. Interwall reactions within
MWNTs can
redistribute current over individual tubes non-uniformly. By contrast, there
is no change in current
across different parts of metallic single-walled nanotubes (SWNTs). Carbon
nanotubes also have
very high thermal conductivity, comparable to diamond crystal and in-plane
graphite sheet.
[00120] The CNT-infused carbon fiber materials can benefit from the presence
of CNTs not
only in the properties described above, but can also provide lighter materials
in the process. Thus,
such lower density and higher strength materials translates to greater
strength to weight ratio. It is
understood that modifications which do not substantially affect the activity
of the various
embodiments of this invention are also included within the definition of the
invention provided
herein. Accordingly, the following examples are intended to illustrate but not
limit the present
invention.
EXAMPLE I
[00121] This example shows how a carbon fiber material can be infused with
CNTs in a
continuous process to target thermal and electrical conductivity improvements.
[00122] In this example, the maximum loading of CNTs on fibers is targeted. 34-
700 12k
carbon fiber tow with a tex value of 800 (Grafil Inc., Sacramento, CA) is
implemented as the carbon
fiber substrate. The individual filaments in this carbon fiber tow have a
diameter of approximately
7 m.
[00123] Figure 8 depicts system 800 for producing CNT-infused fiber in
accordance with the
illustrative embodiment of the present invention. System 800 includes a carbon
fiber material
payout and tensioner station 805, sizing removal and fiber spreader station
810, plasma treatment
station 815, barrier coating application station 820, air dry station 825,
catalyst application station
830, solvent flash-off station 835, CNT-infusion station 840, fiber bundler
station 845, and carbon
fiber material uptake bobbin 850, interrelated as shown.
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[00124] Payout and tension station 805 includes payout bobbin 806 and
tensioner 807. The
payout bobbin delivers carbon fiber material 860 to the process; the fiber is
tensioned via tensioner
807. For this example, the carbon fiber is processed at a linespeed of 2
ft/min.
[00125] Fiber material 860 is delivered to sizing removal and fiber spreader
station 810
which includes sizing removal heaters 865 and fiber spreader 870. At this
station, any "sizing" that
is on fiber 860 is removed. Typically, removal is accomplished by burning the
sizing off of the
fiber. Any of a variety of heating means can be used for this purpose,
including, for example, an
infrared heater, a muffle furnace, and other non-contact heating processes.
Sizing removal can also
be accomplished chemically. The fiber spreader separates the individual
elements of the fiber.
Various techniques and apparatuses can be used to spread fiber, such as
pulling the fiber over and
under flat, uniform-diameter bars, or over and under variable-diameter bars,
or over bars with
radially-expanding grooves and a kneading roller, over a vibratory bar, etc.
Spreading the fiber
enhances the effectiveness of downstream operations, such as plasma
application, barrier coating
application, and catalyst application, by exposing more fiber surface area.
[00126] Multiple sizing removal heaters 865 can be placed throughout the fiber
spreader 870
which allows for gradual, simultaneous desizing and spreading of the fibers.
Payout and tension
station 805 and sizing removal and fiber spreader station 810 are routinely
used in the fiber industry;
those skilled in the art will be familiar with their design and use.
[00127] The temperature and time required for burning off the sizing vary as a
function of (1)
the sizing material and (2) the commercial source/identity of carbon fiber
material 860. A
conventional sizing on a carbon fiber material can be removed at about 650 C.
At this temperature,
it can take as long as 15 minutes to ensure a complete burn off of the sizing.
Increasing the
temperature above this burn temperature can reduce burn-off time.
Thermogravimetric analysis is
used to determine minimum burn-off temperature for sizing for a particular
commercial product.
[00128] Depending on the timing required for sizing removal, sizing removal
heaters may not
necessarily be included in the CNT-infusion process proper; rather, removal
can be performed
separately (e.g., in parallel, etc.). In this way, an inventory of sizing-free
carbon fiber material can
be accumulated and spooled for use in a CNT-infused fiber production line that
does not include
fiber removal heaters. The sizing-free fiber is then spooled in payout and
tension station 805. This
production line can be operated at higher speed than one that includes sizing
removal.
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[00129] Unsized fiber 880 is delivered to plasma treatment station 815. For
this example,
atmospheric plasma treatment is utilized in a `downstream' manner from a
distance of Imm from
the spread carbon fiber material. The gaseous feedstock is comprised of 100%
helium.
[00130] Plasma enhanced fiber 885 is delivered to barrier coating station 820.
In this
illustrative example, a siloxane-based barrier coating solution is employed in
a dip coating
configuration. The solution is `Accuglass T-11 Spin-On Glass' (Honeywell
International Inc.,
Morristown, NJ) diluted in isopropyl alcohol by a dilution rate of 40 to 1 by
volume. The resulting
barrier coating thickness on the carbon fiber material is approximately 40nm.
The barrier coating
can be applied at room temperature in the ambient environment.
[00131] Barrier coated carbon fiber 890 is delivered to air dry station 825
for partial curing of
the nanoscale barrier coating. The air dry station sends a stream of heated
air across the entire
carbon fiber spread. Temperatures employed can be in the range of 100 C to
about 500 C.
[00132] After air drying, barrier coated carbon fiber 890 is delivered to
catalyst application
station 830. In this example, an iron oxide-based CNT forming catalyst
solution is employed in a
dip coating configuration. The solution is `EFH-1' (Ferrotec Corporation,
Bedford, NH) diluted in
hexane by a dilution rate of 200 to 1 by volume. A monolayer of catalyst
coating is achieved on the
carbon fiber material. `EFH-1' prior to dilution has a nanoparticle
concentration ranging from 3-
15% by volume. The iron oxide nanoparticles are of composition Fe2O3 and Fe304
and are
approximately 8 rim in diameter.
[00133] Catalyst-laden carbon fiber material 895 is delivered to solvent flash-
off station 835.
The solvent flash-off station sends a stream of air across the entire carbon
fiber spread. In this
example, room temperature air can be employed in order to flash-off all hexane
left on the catalyst-
laden carbon fiber material.
[00134] After solvent flash-off, catalyst-laden fiber 895 is finally advanced
to CNT-infusion
station 840. In this example, a rectangular reactor with a 12 inch growth zone
is used to employ
CVD growth at atmospheric pressure. 98.0% of the total gas flow is inert gas
(Nitrogen) and the
other 2.0% is the carbon feedstock (acetylene). The growth zone is held at 750
C. For the
rectangular reactor mentioned above, 750 C is a relatively high growth
temperature, which allows
for the highest growth rates possible.
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[00135] After CNT-infusion, CNT-infused fiber 897 is re-bundled at fiber
bundler station
845. This operation recombines the individual strands of the fiber,
effectively reversing the
spreading operation that was conducted at station 810.
[00136] The bundled, CNT-infused fiber 897 is wound about uptake fiber bobbin
850 for
storage. CNT-infused fiber 897 is loaded with CNTs approximately 50 m in
length and is then
ready for use in composite materials with enhanced thermal and electrical
conductivity.
[00137] It is noteworthy that some of the operations described above can be
conducted under
inert atmosphere or vacuum for environmental isolation. For example, if sizing
is being burned off
of a carbon fiber material, the fiber can be environmentally isolated to
contain off-gassing and
prevent damage from moisture. For convenience, in system 800, environmental
isolation is
provided for all operations, with the exception of carbon fiber material
payout and tensioning, at the
beginning of the production line, and fiber uptake, at the end of the
production line.
EXAMPLE II
[00138] This example shows how carbon fiber material can be infused with CNTs
in a
continuous process to target improvements in mechanical properties, especially
interfacial
characteristics such as shear strength. In this case, loading of shorter CNTs
on fibers is targeted. In
this example, 34-700 12k unsized carbon fiber tow with a tex value of 793
(Grafil Inc., Sacramento,
CA) is implemented as the carbon fiber substrate. The individual filaments in
this carbon fiber tow
have a diameter of approximately 7 m.
[00139] Figure 9 depicts system 900 for producing CNT-infused fiber in
accordance with the
illustrative embodiment of the present invention, and involves many of the
same stations and
processes described in system 800. System 900 includes a carbon fiber material
payout and
tensioner station 902, fiber spreader station 908, plasma treatment station
910, catalyst application
station 912, solvent flash-off station 914, a second catalyst application
station 916, a second solvent
flash-off station 918, barrier coating application station 920, air dry
station 922, a second barrier
coating application station 924, a second air dry station 926, CNT-infusion
station 928, fiber
bundler station 930, and carbon fiber material uptake bobbin 932, interrelated
as shown.
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[00140] Payout and tension station 902 includes payout bobbin 904 and
tensioner 906. The
payout bobbin delivers carbon fiber material 901 to the process; the fiber is
tensioned via tensioner
906. For this example, the carbon fiber is processed at a linespeed of 2
ft/min.
[00141] Fiber material 901 is delivered to fiber spreader station 908. As this
fiber is
manufactured without sizing, a sizing removal process is not incorporated as
part of fiber spreader
station 908. The fiber spreader separates the individual elements of the fiber
in a similar manner as
described in fiber spreader 870.
[00142] Fiber material 901 is delivered to plasma treatment station 910. For
this example,
atmospheric plasma treatment is utilized in a `downstream' manner from a
distance of 12mm from
the spread carbon fiber material. The gaseous feedstock is comprised of oxygen
in the amount of
1.1 % of the total inert gas flow (helium). Controlling the oxygen content on
the surface of carbon
fiber material is an effective way of enhancing the adherence of subsequent
coatings, and is
therefore desirable for enhancing mechanical properties of a carbon fiber
composite.
[00143] Plasma enhanced fiber 911 is delivered to catalyst application station
912. In this
example, an iron oxide based CNT forming catalyst solution is employed in a
dip coating
configuration. The solution is `EFH-1' (Ferrotec Corporation, Bedford, NH)
diluted in hexane by a
dilution rate of 200 to 1 by volume. A monolayer of catalyst coating is
achieved on the carbon fiber
material. `EFH-1' prior to dilution has a nanoparticle concentration ranging
from 3-15% by
volume. The iron oxide nanoparticles are of composition Fe203 and Fe304 and
are approximately 8
nm in diameter.
[00144] Catalyst-laden carbon fiber material 913 is delivered to solvent flash-
off station 914.
The solvent flash-off station sends a stream of air across the entire carbon
fiber spread. In this
example, room temperature air can be employed in order to flash-off all hexane
left on the catalyst-
.laden carbon fiber material.
[00145] After solvent flash-off, catalyst laden fiber 913 is delivered to
catalyst application
station 916, which is identical to catalyst application station 912. The
solution is `EFH-1' diluted in
hexane by a dilution rate of 800 to 1 by volume. For this example, a
configuration which includes
multiple catalyst application stations is utilized to optimize the coverage of
the catalyst on the
plasma enhanced fiber 911.
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[00146] Catalyst-laden carbon fiber material 917 is delivered to solvent flash-
off station 918,
which is identical to solvent flash-off station 914.
[00147] After solvent flash-off, catalyst-laden carbon fiber material 917 is
delivered to barrier
coating application station 920. In this example, a siloxane-based barrier
coating solution is
employed in a dip coating configuration. The solution is `Accuglass T-11 Spin-
On Glass'
(Honeywell International Inc., Morristown, NJ) diluted in isopropyl alcohol by
a dilution rate of 40
to 1 by volume. The resulting barrier coating thickness on the carbon fiber
material is
approximately 40nm. The barrier coating can be applied at room temperature in
the ambient
environment.
[00148] Barrier coated carbon fiber 921 is delivered to air dry station 922
for partial curing of
the barrier coating. The air dry station sends a stream of heated air across
the entire carbon fiber
spread. Temperatures employed can be in the range of 100 C to about 500 C.
[00149] After air drying, barrier coated carbon fiber 921 is delivered to
barrier coating
application station 924, which is identical to barrier coating application
station 820. The solution is
`Accuglass T-11 Spin-On Glass' diluted in isopropyl alcohol by a dilution rate
of 120 to 1 by
volume. For this example, a configuration which includes multiple barrier
coating application
stations is utilized to optimize the coverage of the barrier coating on the
catalyst-laden fiber 917.
[00150] Barrier coated carbon fiber 925 is delivered to air dry station 926
for partial curing of
the barrier coating, and is identical to air dry station 922.
[00151] After air drying, barrier coated carbon fiber 925 is finally advanced
to CNT-infusion
station 928. In this example, a rectangular reactor with a 12 inch growth zone
is used to employ
CVD growth at atmospheric pressure. 97.75% of the total gas flow is inert gas
(Nitrogen) and the
other 2.25% is the carbon feedstock (acetylene). The growth zone is held at
650 C. For the
rectangular reactor mentioned above, 650 C is a relatively low growth
temperature, which allows
for the control of shorter CNT growth.
[00152] After CNT-infusion, CNT-infused fiber 929 is re-bundled at fiber
bundler 930. This
operation recombines the individual strands of the fiber, effectively
reversing the spreading
operation that was conducted at station 908.
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[00153] The bundled, CNT-infused fiber 931 is wound about uptake fiber bobbin
932 for
storage. CNT-infused fiber 929 is loaded with CNTs approximately 5 m in length
and is then ready
for use in composite materials with enhanced mechanical properties.
[00154] In this example, the carbon fiber material passes through catalyst
application stations
912 and 916 prior to barrier coating application stations 920 and 924. This
ordering of coatings is in
the `reverse' order as illustrated in Example I, which can improve anchoring
of the CNTs to the
carbon fiber substrate. During the CNT growth process, the barrier coating
layer is lifted off of the
substrate by the CNTs, which allows for more direct contact with the carbon
fiber material (via
catalyst NP interface). Because increases in mechanical properties, and not
thermal/electrical
properties, are being targeted, a `reverse' order coating configuration is
desirable.
[00155] It is noteworthy that some of the operations described above can be
conducted under
inert atmosphere or vacuum for environmental isolation. For convenience, in
system 900,
environmental isolation is provided for all operations, with the exception of
carbon fiber material
payout and tensioning, at the beginning of the production line, and fiber
uptake, at the end of the
production line.
EXAMPLE III
[00156] This example shows how carbon fiber material can be infused with CNTs
in a
continuous process to target improvements in mechanical properties, especially
interfacial
characteristics such as interlaminar shear.
[00157] In this example, loading of shorter CNTs on fibers is targeted. In
this example, 34-
700 12k unsized carbon fiber tow with a tex value of 793 (Grafil Inc.,
Sacramento, CA) is
implemented as the carbon fiber substrate. The individual filaments in this
carbon fiber tow have a
diameter of approximately 7 m.
[00158] Figure 10 depicts system 1000 for producing CNT-infused fiber in
accordance with
the illustrative embodiment of the present invention, and involves many of the
same stations and
processes described in system 800. System 1000 includes a carbon fiber
material payout and
tensioner station 1002, fiber spreader station 1008, plasma treatment station
1010, coating
application station 1012, air dry station 1014, a second coating application
station 1016, a second air
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dry station 1018, CNT-infusion station 1020, fiber bundler station 1022, and
carbon fiber material
uptake bobbin 1024, interrelated as shown.
[00159] Payout and tension station 1002 includes payout bobbin 1004 and
tensioner 1006.
The payout bobbin delivers carbon fiber material 1001 to the process; the
fiber is tensioned via
tensioner 1006. For this example, the carbon fiber is processed at a linespeed
of 5 ft/min.
[00160] Fiber material 1001 is delivered to fiber spreader station 1008. As
this fiber is
manufactured without sizing, a sizing removal process is not incorporated as
part of fiber spreader
station 1008. The fiber spreader separates the individual elements of the
fiber in a similar manner as
described in fiber spreader 870.
[00161] Fiber material 1001 is delivered to plasma treatment station 1010. For
this example,
atmospheric plasma treatment is utilized in a `downstream' manner from a
distance of 12mm from
the spread carbon fiber material. The gaseous feedstock is comprised of oxygen
in the amount of
1.1 % of the total inert gas flow (helium). Controlling the oxygen content on
the surface of carbon
fiber material is an effective way of enhancing the adherence of subsequent
coatings, and is
therefore desirable for enhancing mechanical properties of a carbon fiber
composite.
[00162] Plasma enhanced fiber 1011 is delivered to coating application station
1012. In this
example, an iron oxide based catalyst and a barrier coating material is
combined into a single
`hybrid' solution and is employed in a dip coating configuration. The `hybrid'
solution is 1-part-by-
volume `EFH-1', 5-parts `Accuglass T-11 Spin-On Glass', 24-parts hexane, 24-
parts isopropyl
alcohol, and 146-parts tetrahydrofuran. The benefit of employing such a
`hybrid' coating is that it
marginalizes the effect of fiber degradation at high temperatures. Without
being bound by theory,
degradation to carbon fiber materials is intensified by the sintering of
catalyst NPs at high
temperatures (the same temperatures vital to the growth of CNTs). By
encapsulating each catalyst
NP with its own barrier coating, it is possible to control this effect.
Because increases in mechanical
properties, and not thermal/electrical properties, is being targeted, it is
desirable to maintain the
integrity of the carbon fiber base-material, therefore a `hybrid' coating can
be employed.
[00163] Catalyst-laden and barrier coated carbon fiber material 1013 is
delivered to air dry
station 1014 for partial curing of the barrier coating. The air dry station
sends a stream of heated air
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across the entire carbon fiber spread. Temperatures employed can be in the
range of 100 C to about
500 C.
[00164] After air drying, the catalyst and barrier coating-laden carbon fiber
1013 is delivered
to coating application station 1016, which is identical to coating application
station 1012. The same
`hybrid' solution is used (1-part-by-volume `EFH-1', 5-parts `Accuglass T-11
Spin-On Glass', 24-
parts hexane, 24-parts isopropyl alcohol, and 146-parts tetrahydrofuran). For
this example, a
configuration which includes multiple coating application stations is utilized
to optimized the
coverage of the `hybrid' coating on the plasma enhanced fiber 1011.
[00165] Catalyst and barrier coating-laden carbon fiber 1017 is delivered to
air dry station
1018 for partial curing of the barrier coating, and is identical to air dry
station 1014.
[00166] After air drying, catalyst and barrier coating-laden carbon fiber 1017
is finally
advanced to CNT-infusion station 1020. In this example, a rectangular reactor
with a 12 inch
growth zone is used to employ CVD growth at atmospheric pressure. 98.7% of the
total gas flow is
inert gas (Nitrogen) and the other 1.3% is the carbon feedstock (acetylene).
The growth zone is held
at 675 C. For the rectangular reactor mentioned above, 675 C is a relatively
low growth
temperature, which allows for the control of shorter CNT growth.
[00167] After CNT-infusion, CNT-infused fiber 1021 is re-bundled at fiber
bundler 1022.
This operation recombines the individual strands of the fiber, effectively
reversing the spreading
operation that was conducted at station 1008.
[00168] The bundled, CNT-infused fiber 1021 is wound about uptake fiber bobbin
1024 for
storage. CNT-infused fiber 1021 is loaded with CNTs approximately 2 m in
length and is then
ready for use in composite materials with enhanced mechanical properties.
[00169] It is noteworthy that some of the operations described above can be
conducted under
inert atmosphere or vacuum for environmental isolation. For convenience, in
system 1000,
environmental isolation is provided for all operations, with the exception of
carbon fiber material
payout and tensioning, at the beginning of the production line, and fiber
uptake, at the end of the
production line.
[00170] It is to be understood that the above-described embodiments are merely
illustrative of
the present invention and that many variations of the above-described
embodiments can be devised
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by those skilled in the art without departing from the scope of the invention.
For example, in this
Specification, numerous specific details are provided in order to provide a
thorough description and
understanding of the illustrative embodiments of the present invention. Those
skilled in the art will
recognize, however, that the invention can be practiced without one or more of
those details, or with
other processes , materials, components, etc.
[00171] Furthermore, in some instances, well-known structures, materials, or
operations are
not shown or described in detail to avoid obscuring aspects of the
illustrative embodiments. It is
understood that the various embodiments shown in the Figures are illustrative,
and are not
necessarily drawn to scale. Reference throughout the specification to "one
embodiment" or "an
embodiment" or "some embodiments" means that a particular feature, structure,
material, or
characteristic described in connection with the embodiment(s) is included in
at least one
embodiment of the present invention, but not necessarily all embodiments.
Consequently, the
appearances of the phrase "in one embodiment," "in an embodiment," or "in some
embodiments" in
various places throughout the Specification are not necessarily all referring
to the same
embodiment. Furthermore, the particular features, structures, materials, or
characteristics can be
combined in any suitable manner in one or more embodiments. It is therefore
intended that such
variations be included within the scope of the following claims and their
equivalents.
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